The present invention relates generally to a method of and apparatus for obtaining electrically neutral gas atoms from ionizable gas molecules including the atoms and, more particularly, to such a method and apparatus wherein the gas molecules are converted into a plasma by plural oscillating electromagnetic fields.
It is known that electrically neutral gas atoms can be dissociated from ionizable gas molecules including the atoms by supplying the molecules to a plasma generator including a chamber having an electromagnetic field applied to it. The electromagnetic field ionizes the molecules to form a plasma including positively charged ions, electrons, and dissociated electrically neutral atoms.
Such plasma generators are extensively used on a commercial basis to process workpieces, such as semiconductor wafers, glass panels used in flat panel displays, and metal plates. The plasmas are frequently employed to etch materials from the workpieces or are used in chemical vapor deposition (CVD) processes for depositing materials on the workpieces.
In one class of processors, the electromagnetic field is coupled to the gas in the chamber via a dielectric window from a planar or substantially planar plasma excitation coil located outside the chamber. The coil produces an oscillating RF electromagnetic field coupled into the chamber via the window, which is usually in the chamber roof. The oscillating RF field has magnetic and electric field components that propagate through the dielectric window to heat electrons in the gas in a portion of the plasma in the chamber close to the window. The oscillating RF fields induce in the plasma currents that heat electrons in the plasma. The spatial distribution of the magnetic field in the plasma portion close to the window is a function of the sum of individual magnetic field components produced by each turn of the coil. The magnetic and electric field components produced at each point along the coil are respectively functions of the magnitude of RF current and voltage at each point.
The current and voltage differ for different points because of transmission line effects of the coil at the frequency of the RF source. For spiral-like designs, e.g., as disclosed by Ogle, U.S. Pat. No. 4,948,458, and Holland et al., U.S. Pat. No. 5,759,280, the RF currents in the coil are distributed to produce a torroidal shaped magnetic field region in the portion of the plasma close to the window, which is where power is absorbed by the gas to excite the gas to a plasma. The electric field components start at one portion of the coil, propagate through the window in the chamber and return through the window to a second portion of the coil having a potential different from the first portion.
At pressures greater than 10 millitorr, gas phase collisions of electrons, ions, and neutrons in the plasma prevent substantial diffusion of the plasma charged particles outside the torroidal region. As a result, there is a relatively high plasma density (i.e. flux) in a ring like region below the window but low plasma fluxes below the center and peripheral window portions.
Plasma processing of such workpieces is typically performed at vacuum pressures of between 1 millitorr and 100 millitorr, in relatively large vacuum chambers. The vacuum chambers frequently have a diameter of about 30 centimeters. The workpiece is usually mounted on a chuck located on or in proximity to the chamber floor such that there is typically about a 12 centimeter separation between the coil and workpiece. The plasma in such a chamber typically has a concentration of approximately 109 charge particles per cubic centimeter. The ionized particles and the dissociated atoms are evacuated from the processing chamber by a pump connected to an outlet of the chamber. There is no particular effort to provide a source of dissociated atoms in the effluent evacuated from the chamber.
We, and others, invented an apparatus and method which enables a plasma generator somewhat similar to the prior art plasma chambers excited by RF coils to be used as a source of electrically neutral dissociated atoms. The source supplies the atoms to a downstream workpiece processor, in particular a resist etcher for a semiconductor to wafer. Our copending, commonly assigned, U.S. patent application Ser. No. 09/052,906, filed Mar. 31, 1998, now U.S. Pat. No. 6,203,657, entitled xe2x80x9cInductively Coupled Plasma Downstream Strip Module,xe2x80x9d discloses such an apparatus and method wherein a plasma generator serves as a source of electrically neutral dissociated atoms for a downstream etcher. FIG. 1 is a schematic diagram of such an etcher.
In the arrangement of FIG. 1 gas from a suitable source 8, e.g., oxygen gas (O2) or water vapor (H2O), flows into plasma generator 10 where the gas is converted into an RF plasma including ionized particles, electrons and dissociated neutral atoms (e.g., O resulting from O2 or H2O being dissociated). The dissociated electrically neutral atoms flow from generator 10 into secondary cylindrical chamber 12, thence into cylindrical processor chamber 13 through openings 14 in quartz baffle plate 16. Gas from a second source 9 flows through tube 15 into chamber 12. Processing chamber 13 includes chuck 18 for holding workpiece 20 in situ. RP source 22, typically having a frequency of 13.56 MHz, supplies an RF bias Voltage to chuck 18. Gas in processing chamber 13 is sucked out of the processing chamber by vacuum pump 23 through annular exhaust 24, at the base of chamber 13. The vacuum pump maintains the pressure in generator 10, as well as chambers 12 and 13, in the range of about 500 millitorr to 5 torr.
Plasma generator 10 includes chamber 26 having interior wall surfaces shaped as a right parallelepiped. Chamber 26 has a root formed of dielectric window 28 which carries spiral, substantially planar coil 30, having interior and exterior excitation terminals, as disclosed, e.g., by Ogle, U.S. Pat. No. 4,948,458. The excitation terminals of coil 30 are connected via matching network 31 to be powered by RF plasma excitation source 32, typically having a frequency of 13.56 MHz and an output power of 2 kilowatt. Matching network 31 typically has a T configuration, including a fixed series capacitor, as well as variable series and shunt capacitors having values controlled by a controller (not shown).
The walls and base 34 of chamber 26 are metal, electrically grounded and arranged to include a dielectric, e.g., quartz, liner 36 on the inner surfaces of the chamber. All interior surfaces of chambers 12, 13 and 14, as well as metal grounded conduit 38 connecting chambers 12 and 26, are lined with the same solid dielectric that lines chamber 26. The dielectric liners in chambers 12, 13, 26 and conduit 38 are made of a material that prevents recombination of the oxygen neutral atoms into O2 oxygen molecules and captures charge particles. The gas incident on workpiece 20 typically has a total density of about 1016 atoms per cubic centimeter at a pressure of 1 Torr, and charge particle density of less than 106 charges per cubic centimeter, i.e., the charge particle concentration is less than 1010. In contrast, in commercial prior art processors wherein the plasma generator and the workpiece are in the same chamber, the gas incident on the workpiece typically has a total concentration of about 1014 atoms per cubic centimeter at a 10 millitorr pressure and a charge particle density of about 109 charges per cubic centimeter, i.e., a charge particle concentration of about 105.
Chamber 26 includes opposed end walls 40 and 42 respectively including aligned ports 44 and 46. Conduit 38 includes a bend just downstream of port 46 to assist in capturing a large number of charge particles which are electrically attracted to the metal conduit wall and are captured by the quartz liner in the conduit. The walls of cylindrical chambers 12 and 13 and the roof of chamber 12 are metal at ground potential with a quartz liner, to assist in capturing charge particles in the plasma that plasma generator 10 produces. The same metal plate forms base 34 of chamber 26 and a part of the roof of chamber 12.
Coil 30 responds to RF source 32 to produce an RF electromagnetic field that is coupled through window 28 into chamber 26. The RF electromagnetic field produces a torroidal shaped plasma density pattern 50 including torroidal region 52 of greatest maximum plasma density that occupies a volume about half-way between the peripheral edges 54 and center line 56 of coil 30. Maximum density torroidal region 52 is relatively small and almost abuts window 28. Consequently, only a small portion of the gas flowing through chamber 26 from port 44 to port 46 is converted into the maximum plasma density in region 52. As a result, the amount of gas flowing through port 44 that is converted to a plasma is relatively low. The energy of the plasma maximum density and the maximum electron density of the plasma are also relatively low because of the close proximity of highest plasma density region 52 to the ambient temperature environment outside window 28.
Modeling shows that the density of plasma in chamber 26 corresponds to the torroidal contours 60-66 of FIG. 2. Each of contours 60-66 defines a constant plasma density region in chamber 26. The plasma density between adjacent contours 60-66 increases so that the plasma density within contour 60 is greater than between any pair of adjacent contours. The plasma density between contours 60 and 61 decreases as the spacing from contour 60 increases, and the plasma density outside contour 66 is less than plasma density inside contour 66. The contours of FIG. 2 resulted from modeling a chamber at a pressure of 1 Torr, responsive to O2 flow of 3,000 standard cubic centimeters per minute (SCCM), with a coil supplied with 2 kilowatt at 13.56 MHz.
Torroidal contour 60 of greatest plasma density is very close to dielectric window 28 and displaced considerably from the horizontal center line 69 of chamber 26. Torroidal contour 62 (the contour which intersects horizontal center line 69 and has the greatest plasma density) has a plasma density of about one-half that of the maximum density in contour 60. The close proximity of contour 60 to window 28 results in a substantial amount of plasma energy being lost to the relatively cool wall of the window. Because of all these factors, plasma generator 10 converts a relatively small volume of gas from source 8 into a plasma. As a result, a relatively small percentage of the atoms in source 8 are converted into dissociated, electrically neutral atoms suitable for processing of workpiece 20.
In many applications it is desirable to obtain dissociated electrically neutral atoms, having a concentration as great as possible. We have not been able to obtain the desired high concentrations from the source of FIG. 1. Merely increasing the power applied to coil 30 does not solve the problem because the increased power is likely to result in arcing as a result of high intensity electric fields that occur by increasing power.
It is, accordingly, an object of the present invention to provide a new and improved method of and apparatus for obtaining dissociated electrically neutral atoms of a desired specie from molecules including the atoms, and to a new and improved downstream workpiece processor responsive to such atoms.
Another object of the invention is to provide a new and improved method of and apparatus for deriving a stream of electrically neutral gaseous atoms of a desired specie from a plasma source including an arrangement for supplying electromagnetic fields to gas molecules including the desired specie, and to a new and improved downstream workpiece processor responsive to such atoms.
A further object of the invention is to provide a new and improved method of and apparatus for deriving electrically neutral gaseous atoms having a high concentration, and to a new and improved downstream processor responsive to such atoms.
An additional object of the invention is to provide a new and improved source of electrically neutral gas atoms derived by dissociating gas molecules including the atoms in a plasma generating chamber responsive to an RF coil excitation arrangement, wherein the gaseous species has a high concentration, but the problems associated with breakdown in applying a large amount of power to the electric coil arrangement are substantially avoided, and to a new and improved downstream processor responsive to such atoms.
According to one aspect of the invention, an apparatus for processing a workpiece comprises a source of dissociated electrically neutral gas atoms. The source responds to a gas source including molecules including the atoms and includes a plasma generator for converting gas from the gas source into a gaseous plasma including the gas atoms and ionized particles. The plasma generator includes plural sources for deriving oscillating electromagnetic fields that are coupled to gas from the gas source. The sources of the fields are preferably positioned and arranged for applying different oscillating electric fields for exciting the gas to the plasma to different non-overlapping regions of the gas adjacent opposite boundaries of a chamber through which the gas passes. The source of gas atoms is arranged to capture the ionized particles so that substantially none of the ionized particles can flow through an outlet of the source of gas atoms. The outlet and the chamber included in the plasma generator are arranged so that the electrically neutral gas atoms can flow through the outlet. A processing chamber for the workpiece has an inlet connected to be responsive to gas flowing through the outlet.
Another aspect of the invention concerns a source of electrically neutral gas atoms of a desired specie that responds to molecules having the desired specie. The source comprises a chamber having an inlet connected to a source of the gas molecules. The chamber has a metal wall arrangement and an outlet. The chamber has a window arrangement for coupling oscillating electromagnetic fields from the plural field sources to the interior of the chamber. The oscillating electromagnetic field sources and the windows and the inlet and outlet are positioned so the gas flowing from the inlet to the outlet is excited to a plasma including dissociated electrically neutral gas atoms of the desired specie and charge particles. The field sources are preferably positioned so that electric field components derived from them do not overlap. The walls are arranged and adapted to be connected to a source of electric potential and for capturing substantially all of the charge particles in the plasma to enable the dissociated electrically neutral gas atoms to flow through the outlet without a substantial amount of the charge particles in the plasma.
Preferably each of the sources of oscillating electromagnetic fields includes an RF plasma excitation coil. In the preferred embodiment, first and second of the coils are located adjacent opposed boundaries of the chamber. The coils are positioned outside a solid window arrangement transparent to RF electromagnetic plasma excitation fields from the coils. The window arrangement couples electromagnetic plasma excitation fields from the coils to the gas within the chamber.
In one embodiment the window arrangement includes first and second flat windows that extend in first and second parallel planes on opposite sides of the chamber. The first and second coils are substantially flat and extend in third and fourth planes approximately parallel to the first and second parallel planes. The third and fourth planes are respectively outside the first and second planes. In another embodiment, the window arrangement includes first and second windows having curved cross sections, and the first and second coils respectively have curved cross sections with approximately the same shapes as the curved cross sections of the first and second windows.
Preferably, the plural sources of electromagnetic fields are arranged such that the plasma in the chamber has a density that is substantially symmetrical in amplitude and shape with respect to a center line of the chamber that extends in the same direction as the flow of gas from the gas source to the outlet.
Preferably, an RF excitation arrangement supplies substantially the same amount of RF power to each of the coils, each of which has substantially the same configuration and coupling arrangement with the gas in the chamber.
The chamber outlet includes a wall opposite and in close proximity to an opening in the chamber. The chamber and outlet walls include metal portions connected to a source of electric potential for attracting charge particles in the plasma. The wall arrangement and the outlet wall both include a dielectric liner for preventing recombination of the neutral atoms and for capturing the charge particles of the plasma.
Preferably, the spacing between the electromagnetic field sources is in the range of approximately 2.5 to 5.0 centimeters to provide the desired plasma density and non-overlapping electric field components that do not extend to the chamber center line between the windows.
Another aspect of the invention relates to a method of dissociating molecules into electrically neutral atoms of a desired species. The molecules are provided by a source of an ionizable gas including atoms of a desired species. The method comprises applying different plasma exciting oscillating electromagnetic fields to the gas while it is in a chamber. Each of the electromagnetic fields has electric field components that are preferably applied only to non-overlapping portions of the gas adjacent opposed boundaries of the chamber to form the desired electrically neutral gas atoms of the desired species and ionized gas particles. Substantially all the ionized gas particles are captured in the chamber. The electrically neutral atoms of the desired species flow through an outlet of the chamber.
The plural electric fields components are preferably such that plasma and electron density patterns in the chamber are symmetrical in amplitude and shape with respect to a center line of the chamber. The chamber center line extends in generally the same direction as the direction of flow through the outlet. RF current is supplied to different RF plasma excitation coils to derive the different RF plasma exciting electromagnetic fields, in the preferred embodiment.
Pressure in the chamber is preferably in the range of approximately 500 millitorr to 5 Torr to assist in achieving the desired high concentration of electrically neutral atoms.
We are aware that FIG. 17 of Tobin et al., U.S. Pat. No. 5,619,103, includes an illustration of a plasma processing chamber having a roof and base with dielectric windows, outside of which are respectively located first and second RF plasma excitation coils. The coils supply electromagnetic fields to the interior of the processing chamber to produce plasma for processing top and bottom surfaces of workpieces in the chamber.
There is no indication in the Tobin et al. patent that effluent from the processing chamber does not include a substantial amount of ionized particles. The inference from the patent is that the effluent from the chamber includes a substantial amount of ionized particles as well as electrically neutral atoms since there is no attempt to capture the ionized particles and the effluent merely flows to a vacuum pump. For example, the interior walls of the processing chamber are bare metal and not lined. Since the device disclosed in the Tobin et al. patent appears to be a fairly typical plasma processor wherein the workpiece is in the same chamber as the electromagnetic fields which excite the gas to the plasma, the inference is that the pressure within the chamber is in the conventional range of 1-100 millitorr.
In order for the chamber of the Tobin et al. patent to process workpieces on opposite sides with electromagnetic fields from coils adjacent the processing chamber base and roof, the Tobin et al. processing chamber presumably has the typical height of about 12 centimeters. The distance separating opposite side wall portions of the Tobin et al. chamber is typically at least 25 centimeters. The large volume vacuum chamber of the Tobin et al. patent is not suitable for deriving dissociated electrically neutral gas atoms having a high concentration. The device Tobin et al. discloses in FIG. 17 also appears to be impractical for two-sided workpiece processing since the patent discloses no structure for holding a workpiece in a suspended state between the processing chamber base and roof.
The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed descriptions of several specific embodiments thereof, especially when taken in conjunction with the accompanying drawings.